1. Field of the Invention
This invention relates to the vanes of a turbine assembly and, more specifically, to a ceramic composite vane having a metallic substructure.
2. Background Information
Combustion turbine power plants, generally, have three main assemblies: a compressor assembly, a combustor assembly, and a turbine assembly. In operation, the compressor assembly compresses ambient air. The compressed air is channeled into the combustor assembly where it is mixed with a fuel. The fuel and compressed air mixture is ignited creating a heated working gas. The heated working gas is typically at a temperature of between 2500 to 2900xc2x0 F. (1371 to 1593xc2x0 C.). The working gas is expanded through the turbine assembly. The turbine assembly includes a plurality of stationary vane assemblies and rotating blades. The rotating blades are coupled to a central shaft. The expansion of the working gas through the turbine assembly forces the blades to rotate creating a rotation in the shaft.
Typically, the turbine assembly provides a means of cooling the vane assemblies. The first row of vane assemblies, which typically precedes the first row of blades in the turbine assembly, is subject to the highest temperature of working gas. To cool the first row of vane assemblies, a coolant, such as steam or compressed air, is passed through passageways formed within the vane structure. These passageways often include an opening along the trailing edge of the vane to allow the coolant to join the working gas.
The cooling requirements for a vane assembly can be substantially reduced by providing the vane assembly with a ceramic shell as its outermost surface. Ceramic materials, as compared to metallic materials, are less subject to degrading when exposed to high temperatures. Ceramic structures having an extended length, such as vanes associated with large, land based turbines, are less able to sustain the high mechanical loads or deformations incurred during the normal operation of a turbine vane. As such, it is desirable to have a turbine vane that incorporates a metallic substructure, which is able to resist the mechanical loads on the vane, and a ceramic shell, which is able to resist high thermal conditions.
Prior art ceramic vane structures included vanes constructed entirely of ceramic materials. These vanes were, however, less capable of handling the mechanical loads typically placed on turbine vanes and had a reduced length. Other ceramic vanes included a ceramic coating which was bonded to a thermal insulation disposed around a metallic substructure. Such a ceramic coating does not provide any significant structural support. Additionally, the bonding of the ceramic coating to the thermal insulation precludes the use of a composite ceramic. Additionally, because the ceramic was bonded to the insulating material, the ceramic could not be cooled in the conventional manner, i.e., passing a fluid through the vane assembly. The feltmetal typically has a lower tolerance to high temperature than the metallic substructure, thus additional cooling was required.
Alternative ceramic shell/metallic substructure vanes include vanes having a ceramic leading edge and a metallic vane body, and a rotating blade having a metallic substructure and a ceramic shell having a corrugated metal partition therebetween. These structures require additional assembly steps during the final assembly of the vane or blade which are time-consuming and require a rotational force to activate certain internal seals.
There is, therefore, a need for a composite ceramic vane assembly for a turbine assembly having a metallic core assembly with attached support structures and a ceramic shell assembly.
There is a further need for a composite ceramic vane assembly having a ceramic shell assembly which is structured to be cooled by the cooling system for the vane assembly.
There is a further need for a composite ceramic vane assembly which transmits the aerodynamic forces of the ceramic shell assembly to the metallic core assembly without imparting undue stress to the ceramic shell assembly.
There is a further need for a composite ceramic vane assembly which accommodates differential thermal expansion rates between the ceramic shell assembly and the metallic core assembly while maintaining a positive pre-load on the ceramic shell assembly.
These needs, and others, are satisfied by the invention which provides a turbine vane assembly having a ceramic shell assembly and a metallic core assembly. The metallic core assembly includes an attached support assembly. The metallic core assembly includes passages for a cooling fluid to pass therethrough. The support assembly is structured to transmit the aerodynamic forces of the ceramic shell assembly to the metallic core assembly without imparting undue stress to the ceramic shell assembly. The support assembly can be any one of, or a combination of, a compliant layer, such as a feltmetal, contact points, such as a raised ribs or dimples on the metallic core assembly, or a biasing means, such as a leaf spring.
The metallic core assembly includes at least one cooling passage therethrough. The ceramic shell assembly has an exterior surface, which is exposed to the working gas, and an interior surface. The ceramic shell assembly interior surface is in fluid communication with the metallic core assembly cooling passage. For example, if the ceramic shell assembly is supported by ribs on the metallic core assembly, a cooling fluid may pass between adjacent ribs. If the ceramic shell assembly is supported by a biasing means, the cooling fluid may be passed over the biasing means. If the ceramic shell assembly is supported by a compliant layer, the compliant layer may have cooling passages formed therein.